FIG. 1 illustrates the construction of a conventional cellular system.
As illustrated in FIG. 1, a terminal 1 (101) is located in a service area of a base station (BS) 1(100) and receives service from the BS 1(100).
When the terminal 1(101) moves to a service area of a BS 2(110), the terminal 1(101) performs an inter-cell handover to the BS 2(110). For example, the terminal 1(101) performs a handover to the BS 2(110) as illustrated in FIG. 2.
FIG. 2 illustrates a conventional handover procedure in a cellular system.
As illustrated in FIG. 2, a terminal receiving service from a serving BS obtains information on neighbor BSs from the serving BS (step 200).
After that, the terminal measures the signal quality of the serving BS (step 210), and compares a value of a parameter representing the signal quality of the serving BS with a threshold (step 220).
When the value of the parameter representing the signal quality is less than the threshold, the terminal starts scanning for a handover (step 230). For example, when scanning, the terminal requests the serving BS to perform scanning (step 231). After that, when the terminal receives a scanning performing signal from the serving BS (step 233), the terminal performs scanning for the neighbor BSs using the information on the neighbor BSs provided from the serving BS (step 235). Here, the information on the neighbor BSs includes an identifier of a neighbor BS, a preamble used by neighbor BSs, or training signal information from a physical layer whose signal quality can be measured.
After performing the scanning for the handover, the terminal determines whether to perform a handover to a BS whose signal quality is best with reference to the scanning results (step 240). For example, the terminal determines whether to perform a handover by comparing the sum of a Carrier to Interference and Noise Ratio (CINR) of the serving BS and a handover threshold determined by a target node with a highest CINR confirmed through scanning.
When not performing a handover, the terminal obtains again information on neighbor BSs from the serving BS (step 200).
Meanwhile, when performing a handover to a BS whose signal quality is best, the terminal performs a handover procedure to the BS whose signal quality is best (step 250). For example, when performing a handover to a BS whose signal quality is best, the terminal requests the serving BS to perform the handover (step 251). After that, upon receiving a response signal to the handover request (step 253), the terminal confirms whether a BS accepts the handover from the response signal (step 255).
When the BS rejects the handover (step 258), the terminal receives again a response signal to the handover request (step 253).
Also, when the BS cancels the handover, the terminal suspends the handover.
Meanwhile, when the BS accepts the handover (step 257), the terminal performs network entry to a target BS (step 260). That is, the terminal performs ranging and capability negotiation, an authorization procedure, and a registration procedure with the target BS. Here, the target BS means the BS whose signal quality is best, selected in step 240.
With development of radio technology, small cells such as relay stations (RSs) can be located inside a macro cell as illustrated in FIG. 3.
FIG. 3 illustrates the construction of a conventional multi-hop relay system.
As illustrated in FIG. 3, an RS 1 and an RS 2 are located in a service area of a BS 1, and an RS 3 and an RS 4 are located in a service area of a BS 2 to provide service. Accordingly, a handover by a mobile terminal occurs more frequently than the cellular system illustrated in FIG. 1. For example, when a terminal that has received service from the BS 1 moves to the service area of the RS 1, the terminal performs a handover to the RS 1. Also, when a terminal that has received service from the RS 1 moves to a service area of the RS 2, the terminal performs a handover to the RS 2. Therefore, a handover by a terminal in a multi-hop relay system occurs more frequently than the cellular system illustrated in FIG. 1.
When a handover is performed, an overhead is generated by a terminal due to a ranging procedure, a capability negotiation procedure with a BS accessed through the handover, an authentication procedure, and a registration procedure. Therefore, as occurrence frequency of a handover increases, an overhead for a handover increases even more.